The invention relates to a threaded fastener of the type comprising,.
a shank,
a radially enlarged head at one end of the shank,
an external screw thread on at least a part of the shank,
and a bore extending through the head and into the threaded part of
the shank;
at least the threaded part of the shank being radially expandable into threaded engagement with the wall of a suitable unthreaded bore in a workpiece in which the threaded part has been inserted, the fastener thereafter being removable by unscrewing it from the workpiece bore.
Such a fastener is hereinafter referred to as xe2x80x9ca fastener of the type definedxe2x80x9d. An example of such a fastener is commercially available and well known under the Registered Trade Mark RIVSCREW, and is described in GB 2 140 891 A, to which the reader is referred for a fuller description.
It will be appreciated that when using such fasteners it is the frictional engagement between the fastener and the workpiece which resists and prevents the fastener from becoming unscrewed from the workpiece. There has now arisen a requirement to use such fasteners in workpieces of relatively soft or resilient materials such as certain types of plastics. In such circumstances the frictional engagement may be substantially reduced, due to the resilience or softness of the workpiece material and/or to the reduced co-efficient of friction resulting when one of the surfaces is of plastics material, so that there could be a potential danger of the fastener becoming unintentionally loosened.
Accordingly the present invention provides, in one of its aspects, a fastener he type defined in which the expandable part of the shank is provided with a plurality of radially extending lugs or other protrusions thereby to resist unscrewing the fastener from the workpiece.